You could also mask by AO and then create a texture from masking. Though mask by AO always crashes for me for some reason. If that doesn't work you could try the same thing except with mask by cavity.
Ya mask by AO and mask by Cavity work pretty well, although I generally prefer to create my cavity maps in photoshop using the normal map and the xnormal photoshop plugin. Alot less hassle that way I think
If it didn't work as intended, what as the result and what were you aiming for? You could go to the alpha palette and grab the document. Create a new plane (subdivide but don't smooth) and apply the alpha as a displacement map. Then you could use BPR to render AO, or use MultiMap Exporter to export both an AO and cavity…